Understanding the Open Rear Bulkhead Design Features Essential for the Application of Primer Coatings Using Anodic Electrophoretic Deposition on The Early BMW Neue Klasse Sedans and the BMW 02 Series
This article offers some explanations for specific design features of the 1960s and 1970s BMW Neue Klasse four-door sedans and 02 Series sedans. The features most closely scrutinized include the open design of the rear bulkheads in early Neue Klasse (NK) cars, triangular cutout holes in solid bulkheads of the early 1600-2 02 Series sedans, and various bulkhead and box metal holes in the trunk areas of all NK and 02 Series sedans. The article seeks to explain the design features that enhance the anodic electrodeposition coating process. The author’s comments are specific to the four-door BMW Neue Klasse Sedans, and the BMW 02 Series which are defined as the BMW’s two-door sedans, including the 1600-2, 1602, 1600ti, 2002, 2002ti, 2002tii, 1802, and 1502 models.
Nelson K. Akafuah, Ph.D., Univ. of Kentucky
Adnan Darwish Ahmad, Univ. of Kentucky
During the 1950s and into the early 1960s, entire car bodies were being immersed in vats of primer paint along the assembly line. At first, solvent-based primers were used, which caused environmental hazards and even explosions. For a relatively brief period in the history of automotive primer coating, a water-based primer was used during vat dipping. However, the process still relied on simple gravity to get the primer coated on all parts of the automobile body shell. By the mid-1960s, BMW was using a type of primer coating which utilized electrophoresis in an electrochemical process that would soon be the standard for automotive protective coatings.
In the early years of the BMW Neue Klasse cars, the first years of the 02 Series 1600-2 and 1600ti, and the entire remainder of the 02 Series through 1977—these sedans had some holes and openings in the rear bulkhead (the metal partition behind the rear seat back separating the passenger cabin from the trunk space) as well as holes in other parts of the trunk space. There are very few written documents which explain the purposes of these factory design features, and (thus far) no definitive oral history. This article explores the various rear bulkhead types in these early cars. It explains how these openings enhance what BMW called at the time “Electrophoretic immersion bath” or simply “Electrophoresis.” The process is called by many other names, including electro-coating, electro-painting, electrodeposition, and electrophoretic deposition. Electrophoretic deposition (EPD) is the process where specially formulated primer paint is applied after the phosphate pretreatment of the metal substrate to promote long term corrosion resistance. For the anodic EPD process, the metal body shell carries a positive charge, and a uniquely formulated primer resin carries a negative charge. Thus small paint particles are attracted under electrical influence to a positively charged workpiece (the vehicle). The paint forms an insulating film on the metal, which provides a chemical and physical barrier to prevent corrosion. This paint insulation spreads uniformly to otherwise inaccessible areas of the body, and the distance of such spread is known as “throw power.” Adequate primer coating is highly dependent on open pathways for dispersal of pretreatment liquids and primer throughout the body shell, voltage applied to car body and primer, the paint bath temperature, bath non-volatiles, and vehicle duration (immersion time) in the bath. After immersion, time and temperatures in a drying oven are also critical to the process.
Assembly Line Pretreatment of the Metal Substrate Before the Electrodeposition Process
For optimum protection of the body shell, it imperative to properly prepare the metal surface for the EPD process. The pretreatment allows for a more uniform primer layer. It is chemically driven and in multiple stages, including the following steps: Hot Water Rinsing, Degreasing, Surface Conditioning, and Phosphating Processes.
Other critical process parameters are monitored continuously in the pretreatment bath, including the incoming metal quality, water quality, vehicle immersion time, turnover rates, rinse impingement, and surface flow. Adequate liquid filtration is essential throughout the pretreatment stages to maintain excellent corrosion protection and optimal surface quality. The goal of the pretreatment is to have an optimal phosphate crystal structure and coverage on the metal substrate, getting the car ready for electrochemical primer coating. The authors wish to note that these pretreatment steps also require enough adequate openings in the automotive shell so that pretreatment liquids can reach all portions of the body.
The Anodic Electrophoretic Deposition Process of the 1960s
Anodic EPD technology, introduced in the early 1960s, is an efficient and relatively simple operation with a high degree of automation. The anodic EPD process was subsequently replaced by cathodic technology in the 1970s, the latter which has set new standards in EPD processes and corrosion protection that extends to this day. By 1970, 10% of all vehicles worldwide were electrocoated. By 1990 this had risen to 90% of all vehicles, and today it is by far the most commonly used vehicle coating process (Ansdell, 1999). The liquid primers used in the EPD are often called “electropaints.” Red primers were derived from iron oxide, and grey primers from titanium oxide, mixed with carbon black and yellow iron oxide. In the case of BMW, the company apparently switched from red primer to grey primer sometime around the middle of 1966. Up until 1977, all electropaints used in the automotive industry were of the anodic type, primarily because the resin chemistry was relatively simple, readily available, and adaptable to the needs of the auto industry. In this process, the car was the anode. Figure 1 shows a schematic of the anodic EPD process. The EPD process was reported in great detail by Fettis (Fettis, 1995); a summary is presented here for convenience. A portion of the great success of EDP in the automotive coating process is attributed to throw power. Throw power is a sophisticated yet essential property that defines the ability of an EDP process to deliver primer coating to more remote and harder-to-reach areas. Optimum throw power can depend on a number of technical factors, including pigment volume concentration (which affects film resistance), non-ohmic conductance, rupture voltage, coulomb yield, and neutralizing species type. It is worth mentioning that the typical throw power of the anodic EPD process is about 12-15 cm, while it’s 25-35 cm for the cathodic EPD. The furthest the distance coated and the thickest the film, the better the throw power is.
The anodic process had some deficiencies. The two primary weaknesses were phosphate disruption and poor saponification resistance. Note, saponification is a process by which triglycerides are reacted with sodium or potassium hydroxide (lye) to produce glycerol and a fatty acid salt called soap. The best EPD results include resistance to saponification. During the anodic EPD, very high electrical field forces occur, which rupture many of the metal-phosphate bonds (called phosphate disruption). This leads to a weakening of the adhesion of the phosphate coating to the steel substrate. When subsequent paint coats are applied, stresses are set up in the entire paint film, which, when such damage gets to bare metal, tends to cause the entire paint film to curl away from the damage point. In early anodic systems, as soon as corrosion started at any point of damage, the weakened phosphate layer allowed the paint to peel back, exposing more metal, which was chemically clean and very prone to corrosion. This failure was designated ‘scab corrosion.’ This undesirable property was improved by reducing the incidence of rupture of phosphate bonds by merely increasing their number, i.e., modifying phosphate coatings with densely packed, fine phosphate crystals. Such dense phosphate coatings are of low coating weight, and their introduction led to a marked reduction in the incidence of ‘scab corrosion.’ The chemistry of many early anodic primers was characterized by the use of acid resin chemical systems; therefore, poor saponification resistance was inherent in these electropaint primers.
As a consequence, the deposited cured primer film (based on acid resins), when exposed to an alkaline environment, will tend to form metal soaps soluble in water. When damage occurs to bare metal, caustic salt will simultaneously attack the steel substrate and the electrocoat film, producing rust and the sodium salt (soap) of the anodic resin. This dissolves the primer coating, leading to a loss of adhesion of the remaining paint film and general corrosion problems (Ansdell, 1999).
Electrophoretic Deposition Design Features in BMW Sedans of the Early 1960s
Early 1960s BMW sedans (which introduced the now iconic C-pillar “Hofmeister kink”) began with the Neue Klasse (NK) 1500 sedans, introduced in Frankfurt Motor Show in 1961.
These four-door sedans included the NK 1500 (1962-1964), NK 1800 (1963-1968) NK 1600 (1964-1966), and the NK 2000 and variants (1966-1972). Figure 2 shows a later Neue Klasse sedan in the electrophoretic immersion bath. A photo of the rear bulkhead of a 1963 NK 1500 appears below, the green car (Figure 3). Note the two, vertical oval bulkhead cutouts on each end of the partition, and the rather large, more square openings in the center of the bulkhead. The authors believe this open rear bulkhead design helped facilitate dispersal of pretreatment liquids, as well as primer during the vat dipping process. Similarly, the small hole at the top of the wheelhouse arch is likely a “vent and drain” hole that allows the primer-related liquids to enter the box metal, and also promoting a means for air trapped in the box metal to escape. Also, note the vertical, pressed grooves in the sheet metal on either side of the oval cutouts, as well as in the center dividing metal. The oval cutouts have rolled metal edges. All these intricate pressings and punching of the flat metal is done to give the sheet metal the strength and rigidity needed for such an important structural application. It also appears that this bulkhead relies on five separate metal parts to form the skeleton of the bulkhead partition. Finally, BMW installed a trim board on the rear seat back side of the vehicle as a finishing feature. An early Neue Klasse sedan in the assembly line process after final paint is shown in Figure 4. The open design of the rear bulkhead can be clearly seen against the contrast of the trim board. Note the center metal stanchion separating the two largest open areas, and giving some strength to the mid-sections of the bulkhead.
Sometime in 1964, the rear bulkheads in these NK cars changed from the framed-up cutout design to a solid bulkhead. The trim board was moved outboard of the solid bulkhead, so the trunk had a very finished look. Figure 5 shows a photo of the solid bulkhead with the trim board not yet installed, so you can see a pressed design very similar to what will soon become the familiar rear bulkhead pressing of the 02 Series cars.
At the Geneva Auto Show in March 1966, BMW introduced the first of the company’s new 02 Series, the 1600-2. (“2” was to signify two doors, and a way to distinguish this new model from the four-door NK 1600). From the very first 1600-2 in March 1966, as well as early 1600ti cars, there were unique cutouts, die cut from the pressed sheet metal design on either side of the rear bulkhead. These cutout “triangles” remained in the design until approximately the end of November’s production, 1967.
After that, the bulkhead returned to the solid form in all 02 Series cars, except for a much smaller hole (25 mm), drilled into the solid triangular pressed bulkhead design that had previously been cut out of the 1600-2 cars. Regarding the cars with triangular cutouts, the factory covered these cutouts with acella film (think translucent plastic sheeting) during final fit and finish procedures. Figure 6 shows the inside trunk of a grey car with the factory acella film installed. Note the black wire and the wiring chase at the top of the wheelhouses, this hole goes through to the cabin of the car. At least one additional hole is present on the box metal forming part of the wheelhouse arch in the trunk of all 02 Series cars, from the first 1966 1600-2 to the last 1502 in 1977. These holes were oval shaped and uncovered (Figure 7).
The authors believe BMW designers purposefully included open bulkhead designs in early NK sedans, and the triangular cutouts in early 1600-2 cars, as a way to ensure pretreatment liquids and primer during vat dipping were distributed (as much as possible) throughout the car body.
Other holes in box metal components accomplished the same purpose. Drain holes in the spare tire well were similarly intended to provide adequate drainage of primer liquids, similar to the large drains in the cabin floor of the bodies. Holes cut by the factory in the rear package shelf (some call “rear deck, parcel tray, or hat tray”) also allowed for the passage of liquids associated with the primer process, as well as avenues for air bubbles to escape during vat immersion (See Figure 8). These holes in the rear parcel tray were present for the entire production run of the 02 Series, 1966-1977.
Such holes, openings, drains, and cutouts provided (1) physical access for all liquids associated with the primer process to flow to all areas of the body, and (2) “vent and drain” opportunities for excess primer products and trapped air to exit the car body during the primer coating process. When electrophoretic immersion was added as part of the primer application process, these various openings became all the more important to assist with throw power requirements of EDP in order to get primer into normally inaccessible areas of the car body, and air bubbles out while the car was being immersed.
The authors offer two points of reference that could shed light on the emphasis BMW placed on this relatively new EDP process in marketing the early NK and 02 Series cars. Regarding both the Neue Klasse and 02 Series cars, the YouTube video below depicts the NK/02 Series production line, courtesy of the BMW Group Archive. A full two minutes or more of a 14-minute production documentary is devoted to the electrophoresis process, and how much that process helps the quality of the paint on the finished car.
The second point of reference can be found in early BMW 02 sales brochures (Figure 9 and 10), again touting the electrophoretic immersion bath with words like “All the inaccessible corners and edges of the body, outside reach for normal painting, are also covered with anti-corrosive filler.” Note the BMW shown on this brochure is an early 1600-2, with the triangular bulkhead cutouts and covered with red iron oxide primer.
The photo and text of this brochure exemplify how BMW can get paint to all the nooks and crannies during the vat dip, demonstrating the throw power of the new EPD primer process at the time. Note the sales brochure in Figure 9 is dated May 1968, but the 1600-2 photo is from an earlier car using red primer. The second brochure (Figure 10) is from August 1968 and features a later 02 Series car in grey primer, with the smaller, 25mm holes in the rear bulkhead. The sales brochure in Figure 10 was captioned "Electrophoresis"
Why did these early rear bulkheads finally take on a solid design? For the 02 Series, The Federal Motor Vehicle Safety Standards Act went into effect on January 1, 1968, and mandated solid rear bulkheads as a safety measure to separate the cabin area from the fuel tank on cars. For the Neue Klasse cars, the first impetus to do away with the open rear bulkhead and replace with a solid bulkhead may have been a part of an earlier United Nations agreement which also spoke to fuel safety and cars.
Many European countries, including Germany, were signatory to the United Nations Economic Commission for Europe framework, first signed in March 1958. Parts of this agreement, as amended, cover approximately 147 technical regulations on vehicle safety and environmental concerns. Among the subjects covered are mandatory partitions that separate the passenger compartment from the fuel tank, and standards which limit fuel drip into the passenger cabin under crash conditions.
Given that the agreement allows signatory countries to implement the terms of the UN agreement at various times, it may have been that Germany decided to implement the solid barrier separation between the cabin and fuel tank in 1964. In Europe, this rule is often referred to as the ECE 34 Regulations. The USA never signed the above referenced UN agreement, but rather adopted its own vehicle safety standards, as did Canada. The end result was the Federal Motor Vehicle Safety Standards Act, effective January 1, 1968, mandated the solid rear bulkhead to separate the cabin from the fuel tank area (trunk) in a similar fashion to the earlier UN agreement for European cars. All “US spec” or “Federalized” BMW 1600-2 cars, titled as 1968 models and sold mostly through US dealerships such as the Max Hoffman Corporation, were supposed to have solid rear bulkheads. It’s unclear how many BMW 1600-2 cars, made between September and November of 1967, made it to the US as 1968 Federalized cars with the triangular cutouts in the rear bulk-heads. However, it is believed that no US “Federalized” 1600-2 made after November 1967 had the triangular bulkhead cutouts, nor did any 2002, 1802, 1502—no 02 Series car after approximately November 1967.
Indeed, the final chapter to this “holes in the rear bulkhead” story goes to BMW 02 Series cars made from December 1967 until the end of production. The authors believe every one of these hundreds of thousands of cars (1502, 1602, 1802, 2002, excepting any “Specials” like the Baur cars) had holes drilled in the very same rear bulkhead panels that had been removed in the early 1600-2 cars. In other words, just because some regulations called for diminishment of early, larger “vent and drainage “ triangular openings for primer dispersion and air bubble escape, it looks like BMW did not give up on the notion of having some way to disperse primer to remote areas of the car’s shell, as well as have holes to allow air bubbles to escape during the EPD immersion dipping process. For paint dispersion, it looks like the holes are positioned to make sure paint gets in the inaccessible cable chases at the top of the wheelhouses (where wheelhouse meets bulkhead) and along welding points. These smaller holes in the bulk-head may also have served to supplement larger air escape holes that the factory placed in the rear package shelf (Figure 8).
In a manner similar to the acella film closure finish treatment of the triangular cutouts, these 25mm holes received automotive hole plugs at the end of the assembly line. Some of the plugs in the early cars were very obvious and stood proud from the plane of the bulkhead, as shown in the Figure 11. In other instances (perhaps beginning in 1973 or so) the holes are almost flush with the bulkhead from the trunk side. BMW may have started inserting the automotive hole plugs from the seat back side, or perhaps these are just impressions of holes that the die cutter made but did not quite cut through-and-through. Some owners report no holes at all, but this seems to be rare. Nevertheless, these holes (or vestiges of the holes) stayed in the same triangular press design as was once removed in the early 1600-2 cars and remained until the end of production of the 02 Series. Like the triangular cutouts before them, these holes are placed under the rear package shelf, and may be strategically placed in an area where trapped air bubbles can escape during the immersion dip. If air bubbles are not allowed to escape during the immersion of the car shell in primer, the spots where the bubbles persist will not be adequately covered by primer during e-coating.
As stated above, some holes in the body seem to be designed not only to take in primer, but also to drain primer and expel bubbles after immersion. Examples of these holes are the round holes drilled near the top of the wheelhouse box metal surround on the early Neue Klasse cars (see Figure 3), and the similar, oval shaped holes drilled in every 02 Series car, in the box metal of the wheelhouse arch surround (see Figure 7). Similar holes finished with automotive hole plugs can be found in the spare tire well of all 02 Series cars, presumably for primer drainage. Large holes placed by the factory in the rear parcel shelf are also examples of openings for primer dispersion and air bubble escape (Figure 8).
As one BMW Group Archivist said regarding the purpose of the triangular cutouts, “This is probably one of those things that are lost in time…” However, with some perseverance, a really good circumstantial case can be made for the open rear bulkhead designs, and various holes in the bodies of these early cars being directly related to the application of primer, whether by gravity dipping or EPD technology. The authors believe that EPD primer processes were fully engaged at the Munich-Milbertshofen factory by the mid-1960s, and that use of anodic EPD technology spread to other BMW plants in the following years.
One of the first stops along the way is to see exactly when BMW actually adopted anodic electrophoresis as part of the primer application process. Figures 12, 13, and 14 below, came from the BMW Group Archive. Figure 12 is entitled “BMW electrophoretic dip painting Dip Booth (left) and Drying Oven (right)”. Figure 13 is subtitled “BMW Electrophoresis” and both are dated 1962. This would seem to correspond with the roll-out of the first production Neue Klasse 1500 in 1962. Figure 14 is entitled “Post processing of BMW 1500 after immersion, 1962”. The open rear bulkhead design, and “vent and drain” holes in the wheel housing box metal arch, would seem to be design features for both dispersal of primer to remote interstitial areas (relying too on the extra “throw power” of electrophoretic primer application to get to inaccessible areas of the car’s shell), and holes to open pathways for adequate venting of air bubbles attendant to the immersion process. All of the above notwithstanding, the BMW Group Archive was unable to confirm that the 1962 date on Figures 12, 13, and 14 was an actual date marking the beginning of electrophoretic primer coating, so it could be EPD came to BMW a few years after 1962.
Close examination of period photos from the paint shop portion of the production line of early 02 Series sedans would seem to yield similar evidence that the bulkhead holes were related to electrophoretic primer coating. Figure 15 shows an early 02 1600-2 in the foreground of the electrophoretic immersion bath, with a Neue Klasse car ahead in line. Note, in the photo what appears to be an electrical cable coming out of the center rear window opening and into the trunk space.
Final Thoughts and Disclaimers
The BMW Group Archive could not (with absolute certainty) verify the dates on some of their photographs, even though the dates are on the photos in their archives. Worst case scenario is that the photos of early electrophoresis plant at the Munich-Milbertshofen factory are a bit later than 1962. This would mean that early Neue Klasse cars could have been primer coated in vats and with paint designed for the gravity adhesion process and not the electrophoretic immersion process.
The authors also wish to note that car design often occurred years ahead of assembly line mass production. Since these early 1960s BMW cars were produced at a time when solvent-based and water-based primers utilizing gravity dipping were fading out of the industry, and electrophoretic immersion priming was emerging as the preferred technique, it is possible the newer priming technique simply caught up with an older car design meant for earlier primer techniques. The open bulkheads in early NK cars, and rear bulkhead cutouts of the early 1600-2 sedans, could be vestiges of a design meant to accommodate gravity dipping primers, not electrophoretic dipping. Either way, the authors believe the open construction of the early Neue Klasse cars, and the triangular cutouts in the early 1600-2 cars, are related to design requirements for primer dispersion and air bubble venting (regardless of whether gravity dipping or electro-phoretic deposition primers were used).
The authors wish to thank many people who contributed to this, including a number of people who were contacted through BMW 2002 FAQ. We appreciate your thinking and the photographs contributed for this article. Also thanks so much to Albrecht Walloth of WallothNesch.com, who explained the European laws governing solid rear bulkheads to the writers. Thanks to Chris Bangle, former design chief for BMW, who did not know the answers, but gave a great quote: “Holes are expensive.” Thanks also to Mike Macartney, author of BMW ’02 Restoration Guide, who addressed only the triangular cutouts, but wrote to say “My thoughts are that BMW started off with the cutouts for the reasons you give and then found that there was no need to cut out the sections.” Thanks to Andreas Harz from the BMW Group Archive who wrote several times, but in the end said “This is probably one of those things that are lost in time but if I find something I will tell you.” Big thanks to Ruth Standfuss of the BMW Group Archive, who tirelessly searched for pictures and documents regarding historical records of BMW’s electrophoresis primer endeavors. Thanks to Peter Hope of LVH Coatings Ltd. in the UK, who provided valuable insights as to the triangular cutouts possibly serving as connection points for electrodes, as drain holes for the out-put and input of primer, and the smaller holes being classic venting and draining holes for box metal features.
Any and all comments are welcome.
Ansdell, D. A. (1999). Automotive paints. Paint and surface coatings: theory and practice, 431-489.
Fettis, Gordon, ed. Automotive paints and coatings. VCH Verlagsgesellschaft, Weinheim (Federal Republic of Germany)& VCH Publishers, New York, NY (USA), 1995.
Schrader, H. (1979) BMW, A History. Automobile Quarterly Inc.
About the Authors
Robert P. Smith is a long time sports car fancier and is retired in Hawaii. He has collected and maintained antique sports cars as a hobby for over fifty years. Robert can be reached by personal messaging under the user name @BritshIron through the website BMW 2002 FAQ
Dr. Nelson Akafuah is the Associate Director of the Institute of Research for Technology Development, Univerisity of Kentucky. He is the author of both books and scholarly-refereed journals, including “Evolution of the Automotive Body Coating Process--A Review. “ Dr. Akafuah can be reached at
Adnan Darwish Ahmad is a Ph.D. candidate in the Mechanical Engineering Department at the University
of Kentucky and is under the tutorage of Dr. Akafuah. He is the author of multiple scholarly-refereed
journal articles focused on painting and coating technology. Adnan can be reached at